Dementia can be spotted with a simple eye test years before people begin to lose their memory.

British and Canadian scientists announced the breakthrough at the world’s biggest dementia conference.

More than 850,000 Brits have the condition. Most are only diagnosed in the late stages of the disease when it is too late to do anything about it.

But if the disease is spotted much earlier, lifestyle changes could minimise the extent of the damage.

Drugs are also being developed which experts hope will be able to ward off dementia - but again these will only work if people are diagnosed early.

The latest discovery means opticians could look for signs of dementia during annual eye checks.

Scientists explained that the back of the eye can act as a window to the brain.

It displays signs of changes at the same time as they are taking place deep within the brain circuits - years before symptoms start to appear.

Eye tests could replace expensive brain scans (Image: Getty)

Currently, the only ways to diagnose dementia are expensive brain scans or by puncturing the patient’s spinal cord and taking a fluid sample.

Two research teams told the Alzheimer’s Association International Conference in Toronto about their eye test discovery.

The first team, from London’s Moorfields Eye Hospital in London and Oxford University , found they could use eye scans to measure the precise thickness of a layer of neurons on the retina at the back of the eye.

Scientists measured this layer among 33,000 Brits aged 40 to 69 and also carried out tests on memory, reaction time and reasoning.

“This provides a possible new biomarker for studies of neurodegeneration.”

High-tech imaging can spot amyloid deposits in the eye (Image: Getty)

Dr Simon Ridley, director of research at Alzheimer’s Research UK, said: “Non-invasive tests such as this could allow ophthalmologists to aid in the identification of people at risk of memory decline who could benefit from more close assessment and support over time.”

A second study, by scientists at Waterloo University in Canada, found people with severe Alzheimer’s disease had deposits of a protein called amyloid on their retinas.

Amyloid, thought to be one of the key causes of Alzheimer’s, which makes up the majority of dementia cases, is often found to have formed into clumps and plaques in the brain.

This is the first time scientists have found it is also visible on the eye.

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Researchers examined the eyes and brains of 20 humans who had died with Alzheimer’s, along with the eyes of six dogs displaying symptoms of the disease.

The brains were full of amyloid plaques, as expected, but the researchers found they could also spot amyloid deposits in the eye using high-tech imaging.

Scientists believe the technique could easily be used by opticians on living patients.

Dementia affects hundreds of thousands of people in the UK (Image: Getty)

Researcher Dr Melanie Campbell said: “With its neural tissue the eye is an extension of the brain. Unlike current brain scans and cerebo-spinal fluid testing for amyloid, this is a comfortable and potentially widely available test for early detection.”

The researchers now aim to adapt standard opticians’ instruments to incorporate the imaging techniques.

Dr Clare Walton, research manager at the Alzheimer’s Society, said: “Changes in the brain associated with dementia can begin several years before any memory symptoms appear. This research suggests some of these changes happen in the retina of the eye too, which could give us a relatively easy, non-invasive way to spot them early.

“Eye tests are fairly common for older people, so there is great potential to incorporate additional tests into their regular check-up.

“These tests could help to identify people at risk of dementia who would benefit from further investigation but will not become a primary way to diagnosis the condition.”

Dr Ridley added: “Being able to detect changes in amyloid using non-invasive techniques could be a promising development in the search for new ways to understand its role in Alzheimer’s and identify those at risk of the disease.”